Low density lipoprotein (LDL) and β-very low density lipoprotein (β-VLDL) are internalized by the same receptor in mouse peritoneal macrophages and yet their endocytic patterns differ; β-VLDL is targeted to both widely distributed and perinuclear vesicles, whereas LDL is targeted almost entirely to perinuclear lysosomes. This endocytic divergence may have important metabolic consequences since β-VLDL is catabolized slower than LDL and is a more potent stimulator of acyl-CoA/cholesterol acyl transferase (ACAT) than LDL. The goal of this study was to explore the determinants of β-VLDL responsible for its pattern of endocytic targeting. Fluorescence microscopy experiments revealed that large, intestinally derived, apoprotein (Apo) E-rich β-VLDL was targeted mostly to widely distributed vesicles, whereas small, hepatically derived β-VLDL was targeted more centrally (like LDL). Furthermore, the large β-VLDL had a higher ACAT-stimulatory potential than the smaller β-VLDL. The basis for these differences was not due to fundamental differences in the means of uptake; both large and small β-VLDL were internalized by receptor-mediated endocytosis (i.e., not phagocytosis) involving the interaction of Apo E of the β-VLDL with the macrophage LDL receptor. However, large β-VLDL was much more resistant to acid-mediated release from LDL receptors than small β-VLDL. Furthermore, partial neutralization of the multiple Apo Es on these particles by immunotitration resulted in a more perinuclear endocytic pattern, a lower ACAT-stimulatory potential, and an increased sensitivity to acid-mediated receptor release. These data are consistent with the hypothesis that the interaction of the multivalent Apo Es of large β-VLDL with multiple macrophage LDL receptors leads to a diminished or retarded release of the β-VLDL from its receptor in the acidic sorting endosome which, in turn, may lead to the widely distributed endocytic pattern of large β-VLDL. These findings may represent a physiologically relevant example of a previously described laboratory phenomenon whereby receptor cross-linking by multivalent ligands leads to a change in receptor targeting.
ASJC Scopus subject areas
- Cell Biology